Diagnostic apparatus for exhaust gas clarification apparatus for internal combustion engine

ABSTRACT

A diagnostic apparatus for exhaust gas clarification apparatus for internal combustion engine is disclosed. The diagnostic apparatus can diagnoses the individual catalyst independently by means that an air/fuel ratio sensor is mounted before and after the individual catalyst, and that the individual catalyst is diagnosed by estimating the correlation of the output signals from the air/fuel ratio sensors mounted before and after the individual catalyst, and furthermore, that the individual diangosis operation is performed at distinctive operation regions adequate for their corresponding diagnostic mode.

BACKGROUND OF THE INVENTION

The present invention relates to a diagnostic apparatus for an exhaustgas clarification apparatus of an internal combustion engine using aplurality of catalyst converters arranged in series configuration, morespecifically, to a diagnostic apparatus for an exhaust gas clarificationapparatus of an internal combustion engine which diagnoses a status ofthe clarification apparatus by judging output signals from a pluralityof air/fuel ratio sensors installed in the upper stream and the downstream of said plurality of catalyst converters.

An apparatus for clarifying the combustion exhaust gas in the internalcombustion engine is generally composed of catalyst converters and anair/fuel ratio feedback control apparatus. A catalyst converter ismounted in the exhaust pipe for removing hazardous components in theexhaust gas, specifically including HC, NOx and CO. An air/fuel ratiofeedback control apparatus is aimed to keep the air/fuel ratio constantin order to make the best use of function of catalyst converters, andcontrols the amount of fuel supplied to the engine in responsive to theair/fuel ratio which is obtained with an air/fuel ratio sensor or anoxygen sensor mounted on the upper stream of the catalyst converter inthe exhaust gas pipe.

The three-way catalyst used in the above described catalyst convertershas such a problem that the clarification performance is degraded evenwithin the durable time predefined based on the expectedaged-deterioration because the impurity materials stick to the catalystpart of the catalyst converters when supplying lead gasoline to theinternal combustion engine as fuel.

As for the above described problem, in such a proposed system asdisclosed in Japanese Laid-Open Patent 97852(1988), a secondary exhaustgas sensor for detecting oxygen density is so located in the down streamside of the catalyst converter in the exhaust system of the internalcombustion engine as to judge the degradation of catalyst operationperformance of the catalyst converter by referring to the number ofaltering output signals in the air/fuel ratio feedback control with thesecondary exhaust gas sensor. In this proposed system, what is aimed isthat there is no difference between the oxygen density in the downstream side of the catalyst and that in the upper stream of the catalystdue to the decrease in the oxygen absorption power of the catalyst whenthe catalyst operation performance is degraded, and that thisindifference is used for judging indirectly the degradation of thecatalyst.

As for the prior art related to the diagnosis of plurality of catalystsarranged in series in the exhaust gas system, an example is disclosed inJapanese Laid-Open Patent 26032(19993). In this prior art system, asecondary (second) exhaust gas sensor is installed in the in-betweenpart between the upper stream catalyst converter and the down streamcatalyst converter in the exhaust gas system, and a tertiary (third)exhaust gas sensor is installed in the down stream part of the downstream catalyst. In this system, under the air/fuel ratio feedbackcontrol based on the primary (first) exhaust gas sensor, the degradationof the upper stream catalyst is judged by referring to the number ofinversions in the second exhaust gas sensor. In addition, under theair/fuel ratio feedback control based on the second exhaust gas sensor,the degradation of the upper stream catalyst is judged by referring tothe number of inversions in the third exhaust gas sensor.

In such a prior art as having a plurality of catalysts arranged inseries in the exhaust gas system, the following problem arises whensimply applying a configuration including an exhaust gas sensor locatedin the down stream side of the down stream catalyst converter andjudging the degradation of the catalyst based on the changes in theoutput signal from this sensor.

As the components and their individual concentration of the combustionexhaust gas supplied to the upper stream catalyst converter is differentfrom those of the gas supplied to the down stream catalyst converter,the degree of degradation varies with the individual catalystconverters, which leads to the difficulty in keeping stable reliability.

Regarding to the above problem, what may be considered is such asolution that an exhaust gas sensor for judging the degradation of theupper stream catalyst is mounted in the in-between part between theupper stream catalyst converter and the down stream catalyst converterin the exhaust gas system. However, as larger amount of oxygen in theexhaust gas is absorbed in the upper stream and down stream catalystconverters compared with the case of using a single unit of catalystconverter, the partial pressure of oxygen in the combustion exhaust gasin the down stream side of the down stream catalyst converter does notchange extremely when the air/fuel ratio feedback control is based onthe output signal from the exhaust gas sensor located in the upperstream side of the upper stream catalyst converter. Under thiscircumstance, any explicit change in the output signal from the exhaustgas sensor located in the down stream side of the down stream catalystconverter can not be seen other than the case that the catalystperformance is extremely degraded, which leads to the problem that theaccuracy in degradation judgment of the down stream catalyst convertermay be reduced.

Thus, in the prior art, as an attempt is made to solve above describedproblems by applying an air/fuel ratio feedback control based on theoutput signal from the exhaust gas sensor installed in the in-betweenpart of the upper stream and down stream catalysts. However, as enginecontrol parameters are forced to be changed for diagnostic operationsfor the degradation of catalyst, another new problem including by-effectto exhaust system characteristic and performance should be considered.

SUMMARY OF THE INVENTION

An object of the present invention is, especially in an exhaust gasclarification apparatus of the internal combustion engine having aplurality of catalysts, to provide a diagnostic system enabling todiagnoses the individual catalyst independently by means that anair/fuel ratio sensor is mounted before and after the individualcatalyst, and that the individual catalyst is diagnosed by estimatingthe correlation of the output signals from the air/fuel ratio sensorsmounted before and after the individual catalyst, and furthermore, thatthe individual diagnosis operation is performed at distinctive operationregions adequate for their corresponding diagnostic mode.

The diagnostic apparatus of the exhaust gas clarification apparatus ofthe internal combustion engine comprises a plurality of exhaust gasclarification catalysts in the exhaust gas route in the internalcombustion engine having a means for detecting the operation status ofthe internal engine, and an air/fuel ratio control means for regulatingthe amount of fuel injection so as to keep the constant air/fuel ratioin the exhaust gas; the first air/fuel ratio sensor located in the upperstream of the upper stream catalyst in the exhaust gas route; the secondair/fuel ratio sensor located between the upper stream catalyst and thedown stream catalyst in the exhaust gas route; the third air/fuel ratiosensor located in the down stream of the down stream catalyst; adiagnostic means for performing the diagnosis of the upper streamcatalyst based on the output signals from the first air/fuel ratiosensor and the second air/fuel ratio sensor; and further a diagnosticmeans for performing the diagnosis of the overall catalyst apparatusincluding the upper stream catalyst and the down stream catalyst byusing the first air/fuel ratio sensor and the third air/fuel ratiosensor; and a means for diagnosing the down stream catalyst in theexhaust gas route based on the diagnostic information of the upperstream catalyst in the exhaust gas route and the overall catalystapparatus, that is, a means for estimating the degree of degradation ofthe down stream catalyst in the exhaust gas route by referring to thedata map which is formed as a two dimensional matrix defined by a coupleof axis representing the diagnosis information of the upper streamcatalyst and the diagnosis information of the overall catalystapparatus.

In diagnostic apparatus of the exhaust gas clarification apparatus ofthe internal combustion engine in the present invention, the internalcombustion engine operation region adequate for the diagnosis of theupper stream catalyst in the exhaust gas route and the internalcombustion engine operation region adequate for the diagnosis of theoverall catalyst apparatus are defined as distinctive regions, and thediagnosis of the overall catalyst apparatus is performed in theoperation region with higher load than the operation region for thediagnosis of the upper stream catalyst in the exhaust gas route.

In diagnostic apparatus of the exhaust gas clarification apparatus ofthe internal combustion engine in the present invention, in case thatthe difference between the clarification power of the upper streamcatalyst and that of the down stream catalyst is larger than adesignated value, the diagnosis of the down stream catalyst is performedbased on both the output signals from the first air/fuel ratio sensorand the third air/fuel ratio sensor, and the output signals from thesecond air/fuel ratio sensor and the third air/fuel ratio sensor, oreither one of the output signals.

The catalyst diagnostic method of the diagnostic apparatus of theexhaust gas clarification apparatus of the internal combustion engine soconfigured as in above description in the present invention uses thecorrelation method for obtaining the correlation between the outputsignals from the air/fuel ratio sensors mounted before and after thecatalyst. This method uses the principle that, if the upper streamcatalyst or the down stream catalyst is not degraded, the change in theair/fuel ratio becomes smaller in the down stream of the catalyst due tothe oxidation and reduction operation by the catalyst, and therefore thechanges in the detected signal of the air/fuel ratio sensor in the downstream changes less; on the other hand, if the catalyst apparatus isdegraded, the change in the air/fuel ratio in the down stream catalystis much closely correlated to the change in the air/fuel ratio in theupper stream catalyst. The degradation of the catalyst is diagnosed withthe above point of view related to the similarity of changes in theair/fuel ratio measured before and after the catalyst apparatus.

As a measure representing the similarity of changes in the air/fuelratio measured before and after the catalyst apparatus, thecross-correlation function of the detected signal from the air/fuelratio sensors before and after the catalyst apparatus is calculated. Thehigher the similarity of changes in the air/fuel ratio, that is, theoutput signals of the air/fuel ratio sensor measured before and afterthe catalyst apparatus, the higher the value of the cross-correlationfunction.

By cross-correlation function calculation based on the correlationmethod, the cross-correlation function value of the output signals fromthe first air/fuel ratio sensor and the second air/fuel ratio sensor iscalculated, and then, the degradation of the upper stream catalyst isdiagnosed. And, by correlation function calculation means, thecorrelation function value of the first air/fuel ratio sensor and thethird air/fuel ratio sensor is calculated, and then, the degradation ofthe overall catalyst apparatus including the upper stream catalyst andthe down stream catalyst is diagnosed.

The calculation of the individual correlation function is performedafter verifying that the operation status of the internal combustionengine is located in the catalyst diagnosis region.

In the diagnostic apparatus of the exhaust gas clarification apparatusof the internal combustion engine in the present invention, a by-passroute for by-passing the exhaust gas from the upper stream catalyst, anda by-pass valve for switching the exhaust gas flow into the upper streamcatalyst or into the by-pass route are installed, and also with thesecond air/fuel ratio sensor is located at the upper stream part or thedown stream part of the outlet port of the by-pass route, the problemrelated to the reduction of reliability in diagnosis for the down streamcatalyst can be resolved, and the independent and individual diagnosisof the upper stream catalyst and the down stream catalyst is enabled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a basic schematic diagram of the diagnostic apparatus of theexhaust gas clarification apparatus of one embodiment in the presentinvention.

FIG. 2 is a control flow chart of the embodiment shown in FIG. 1.

FIG. 3 is a basic schematic diagram the diagnostic apparatus of theexhaust gas clarification apparatus shown in FIG. 1 with its catalystdiagnosis part modified.

FIG. 4 is a basic schematic diagram of the diagnostic apparatus of theexhaust gas clarification apparatus of another embodiment in the presentinvention.

FIG. 5 is a control flow chart of another embodiment shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a basic schematic diagram of the diagnostic apparatus of theexhaust gas clarification apparatus for the internal combustion enginein the present invention.

In the system of this embodiment, the amount of intake air passingthrough the air cleaner and the throttle valve, both not shown, andthrough the throttle sensor 2 for detecting the throttle angle ismeasured by the air flow sensor 1. The control apparatus 3 of theinternal combustion engine calculates an optimal amount of fuelinjection by judging the measured values from the air flow sensor 1 andthe rotation sensor not shown, and controls the internal combustionengine so that the fuel may be injected by the injector 4 based on thecalculated values.

The mixed gas of air and fuel so obtained by the measurement and thecalculation as shown above is taken through the intake manifold 5 intothe inner chamber of the combustion chamber of the inner combustionengine E, and is exhausted after completing the processes ofcompression, explosion and expansion of the inner combustion engine E.The first air/fuel ratio sensor 6 is made to be mounted in the midst ofthe exhaust manifold 5' or at the end of the exhaust manifold 5' inorder to detect the density of oxygen in the exhaust gas and the amountof fuel injection from the injector 4 is corrected by the detectedsignal of the first air/fuel ratio sensor 6. In the system of thisembodiment, what is established is a catalyst apparatus including theupper stream catalyst converter 9 and the down stream catalyst converter10, both so arranged in series as to be served as a clarification meansfor combustion exhaust gas.

The second air/fuel ratio sensor 7 is installed in the down stream ofthe upper- stream catalyst converter 9, and the third air/fuel ratiosensor 8 is installed in the down stream of the down stream catalystconverter 9. Though an oxygen sensor is used as an air/fuel ratio sensorin this embodiment, it may be allowed to use another type of air/fuelratio sensor.

Each part of the catalyst diagnostic part 11 of the system in thisembodiment is described. An input signal describing the operation statusof the internal combustion engine is supplied to the catalyst diagnosisregion judging means 12 in which whether the operation status of theengine is located in the region adequate for rational judgment of thecatalyst converters is judged.

In catalyst diagnosis for the catalyst apparatus in this embodiment, acorrelation method for the correlation between the output signals of theair/fuel ratio sensors installed before and after the catalyst. In casethat the upper stream catalyst converter 9 or the down stream catalystconverter 0 is not degraded, the change in the air/fuel ratio at thedown stream of the catalyst apparatus becomes smaller owing to theoxidation and reduction operation by the catalyst, which leads to thesmaller change in the detected signal by the air/fuel ratio sensor 8 atthe down stream. On the other hand, in case that the catalyst becomedegraded, the change in the air/fuel ratio at the upper stream becomescloser to that in the air/fuel ratio at the down stream. Thus, theprinciple of diagnosis of the degradation of the catalyst is to considerthe similarity between the changes in the air/fuel ratio before andafter the catalyst.

As a measure representing the similarity between the changes in theair/fuel ratio before and after the catalyst, the cross-correlationfunction of the detected signals from the air/fuel ratio sensors, oneinstalled before the catalyst apparatus and the other installed afterthe catalyst apparatus. The correlation function takes a larger value ifthe similarity between the changes in the air/fuel ratio before andafter the catalyst is larger, and on the other hand, the function takesa smaller value if the similarity is smaller.

Any other measure which could represent the similarity between thechanges in the air/fuel ratio before and after the catalyst can beapplicable other than the value of cross-correlation function in orderto evaluate the degradation of the catalyst.

By cross-correlation function calculation means 13 using the correlationmethod, the correlation function value of the output signals from thefirst air/fuel ratio sensor 6 and the second air/fuel ratio sensor 7 iscalculated, and the degradation diagnosis of the upper stream catalystconverter 9 is performed with the calculated value. By cross-correlationfunction calculation means 14 using the correlation method, thecorrelation function value of the output signals from the first air/fuelratio sensor 6 and the third air/fuel ratio sensor 8 is calculated, andthe degradation diagnosis of the overall catalyst apparatus includingthe upper stream catalyst converter 9 and the down stream catalystconverter 10 is performed with the calculated value.

The calculation of the individual correlation functions is started whenthe catalyst diagnosis region judging means 12 judges that the operationstatus of the internal combustion engine is located within the catalystdiagnosis region. The conditions used for this judgment include at leastthe number of engine rotations, load, the amount of intake air, thefeedback condition for the air/fuel ratio, the catalyst temperature andso on.

The cross-correlation function calculation means 13 and thecross-correlation function calculation means 14 have at least a samplingmeans for the output signals from the air/fuel ratio sensors 6, 7 and 8,and a signal processing means for the output signals from the air/fuelratio sensors 6, 7 and 8.

And next, in the down stream catalyst diagnosis means 15, the degree ofdegradation of the down stream catalyst 10 is estimated by the diagnosisresult of the upper stream catalyst 9 and the diagnosis result of theoverall catalyst apparatus. The estimation method of the degree ofdegradation of the down stream catalyst 9 uses a value referred from thedegradation map for the down stream catalyst 10 which is formed as a twodimensional matrix defined by a couple of axis representing thediagnosis result of the upper stream catalyst 9 and the diagnosis resultof the overall catalyst apparatus 10.

The individual calculated value of the correlation functions is suppliedto the degradation judging means 16, and is compared with the thresholdvalue prepared for judging the degree of degradation for the individualtype of catalyst, and in case that the calculated value is proved to belarger than its corresponding threshold value, the catalyst is judged tobe degraded. The operation status of the engine when the diagnosis isundergone is supplied into the degradation judging means 16, and thejudging result based on the operation status is also corrected. Inaddition, in case that the catalyst is judged to be degraded with thememorized result of the above described judgment, the failure of thecatalyst is reported, for example, by lightning an alarm display lamp.

One of the characteristic points of the diagnostic system in thisembodiment is to performs the individual calculations of the correlationfunctions at their distinctive operation regions rather thansimultaneously. For example, the diagnosis of the upper stream catalyst9 is performed in an operation region in which the engine is operatedwith relatively lower load, and the diagnosis of the overall catalystapparatus is performed in an operation region in which the engine isoperated with higher load. Thus, optimal conditions for the individualcatalysts can be established with applying different operation regionsto the individual diagnostic procedures.

FIG. 2 is a control flow chart of the diagnostic system in thisembodiment, and will be described below.

When the internal combustion engine is started, the control apparatus 3is initiated for starting the control operation, and the control programstarts at first with step S201 where whether the operation region fordiagnosis of the upper stream catalyst 9 is established or not isjudged. If this region is proved to be established, step S202 isselected as the next step, and next, in step S202 the calculation of thecorrelation function is performed by the correlation functioncalculation means 13 based on the output signals from the first air/fuelratio sensor 6 and the second air/fuel sensor 7.

If step S201 concludes that the operation region is not established,step S203 is selected as the next step, where whether the operationregion for the diagnosis of the overall catalyst system is establishedor not is judged by the catalyst function calculation means 12. In casethat the operation region is proved to be established, step S204 isselected as the next step, where the calculation of the correlationfunction is performed by the correlation function calculation means 14based on the output signals from the first air/fuel ratio sensor 6 andthe third air/fuel sensor 8.

If step 203 concludes that the operation region for diagnosis of theoverall catalyst apparatus is not established, step S210 is selected forgoing back to the initial step of the flowchart and restarting thecontrol program.

When completing the diagnosis of the upper stream catalyst 9 and thediagnosis of the overall catalyst apparatus, further step S205 isselected next, where the individual diagnostic results of the diagnosisof the upper stream catalyst 9 and the diagnosis of the overall catalystapparatus are supplied, and the degree of degradation of the down streamcatalyst is estimated by using a data map formed as a two dimensionalmatrix defined by a couple of axis representing the diagnosis result ofthe upper stream catalyst 9 and the diagnosis result of the overallcatalyst apparatus 10. And next, going forward to step S206, abovedescribed three sets of diagnostic results and the estimated value forthe degree of degradation of the down stream catalyst are stored in thememory, and then step S207 is selected as the next step.

In step S207, the diagnostic result of the upper stream catalyst 9, thediagnostic result of the overall catalyst apparatus, and the estimatedvalue for the degree of the degradation of the down stream catalyst arecompared with their corresponding threshold value for degradationjudgment, respectively. In case that any one of the diagnostic resultsso obtained above is judged to exceed the threshold value, step S208concludes that its corresponding catalyst is degraded, and next, stepS209 is selected as the next step. In step S209, the fact that thespecific catalyst is degraded is reported to the driver.

In case that all of the diagnostic results so obtained above are judgednot to exceed the threshold value, step S210 is selected for going backto the initial step of the flowchart and restarting the control program.

The diagnostic system as shown in FIG. 3 is assumed to be applied to thecase that the difference between the clarification ability of the upperstream catalyst 9 and that of the down stream catalyst 10 is emerged(for example, the clarification power of the down stream catalyst 10 islarger than that of the upper stream catalyst 9). In this diagnosticsystem, the diagnosis of the down stream catalyst is made possible byusing the correlation between the output signals from the first air/fuelratio sensor 6 and the third air/fuel ratio sensor 8, or the correlationbetween the output signals from the second air/fuel ratio sensor 7 andthe third air/fuel ratio sensor 8. The diagnostic method of thediagnostic system shown in FIG. 1 can be also applied to this diagnosticsystem shown in FIG. 3. As the diagnosis of the down stream catalyst 10can be performed directly, in the diagnostic system shown in FIG. 3, thedown stream catalyst diagnosis means 15 is not necessary as well as thedegradation degree estimation map for the down stream catalyst 10 isnot. In case that it is necessary to obtain the degradation status ofthe overall catalyst apparatus, this can be estimated by using theindividual diagnostic results of the upper stream catalyst 9 and thedown stream catalyst 10. For example, this estimation calculation mayuse a degradation status data map formed as a two dimensional matrixdefined by a couple of axis representing the diagnosis result of theupper stream catalyst 9 and the diagnosis result of the overall catalystapparatus 10.

The diagnostic system shown in FIGS. 1 and 3 has a means for correctingthe air/fuel ratio feedback control based on the output signal from theair/fuel ratio sensor 7 or performing the air/fuel ratio feedbackcontrol based on the air/fuel ratio sensor 7 when the correlation valueof the output signals from the air/fuel ratio sensors 6 and 7 mountedbefore and after the upper stream catalyst respectively.

FIG. 4 is a schematic diagram of the diagnostic apparatus in anotherembodiment of the present invention. The major difference from thediagnostic system described in the previous embodiment is to arrange theby-pass valve 17 and the by-pass route 18 for by-passing the exhaust gasdirectly to the down stream catalyst. In FIG. 4, similar parts and meansare designated identical numbers which are not explained here.

In the diagnostic system of this embodiment, the by-pass valve 17 ismade to open when performing the diagnosis of the down stream catalyst10 in order to allow the total volume of the exhaust gas from the engineto pass through the by-pass route 18 and not to go into the down streamcatalyst 10. With this configuration, the diagnosis of the down streamcatalyst can be performed in the manner similar to that for the singlecatalyst structure. In this case, the diagnostic condition for the downstream catalyst 10 assumes that the down stream catalyst 10 is fullyactivated or that the exhaust gas does not suffer from flow obstacle orloss physically. Though the second air/fuel ratio sensor 7 is located inthe upper stream side of the outlet port of the by-pass route 18 in FIG.4, it is allowed to arrange the second air/fuel ratio sensor 7 in thedown stream side.

The function of the diagnostic system of this embodiment includes thecontrol of the by-pass valve 17 so as to open in the diagnostic mode aswell as in protecting the upper stream catalyst 9 from high temperaturegas flow in the high-speed and high-load engine operations.

In the similar manner to the first embodiment, also in the diagnosticsystem of this embodiment, the upper stream catalyst 9 is diagnosedbased on the correlation function of the output signals from the firstair/fuel ratio sensor 6 and the second air/fuel ratio sensor 7, and thedown stream catalyst 10 is diagnosed based on the correlation functionof the output signals from the first air/fuel ratio sensor 6 and thethird air/fuel ratio sensor 8. Exceptionally in case that the secondair/fuel ratio sensor 7 is located in the down stream side of theby-pass route 18, it is allowed to diagnose the down stream catalyst 10based on the correlation of the output signals from the second air/fuelratio sensor 7 and the third air/fuel ratio sensor 8. In case of thediagnosis using the correlation between the output signals from thesecond air/fuel ratio sensor 7 and the third air/fuel ratio sensor 8, asthe second air/fuel ratio sensor 7 is so located close enough to thedown stream catalyst 10, the time lag between the signal sensing by bothair/fuel ratio sensors 7 and 8 can be reduced, and therefore, moreprecise correlation can be obtained.

FIG. 5 is a control flow chart of the diagnostic system of thisembodiment, and will be described below.

When the internal combustion engine is started, the control apparatus 3is initiated for starting the control operation, and the control programstarts at first with step S401 where whether the operation region fordiagnosis of the upper stream catalyst 9 is established or not isjudged. If this region is proved to be established, step S402 isselected as the next step. What is judged in step S402 is whether theby-pass valve 17 is OFF or not, that is, the by-pass route 18 is closedor not. If step S402 concludes that the valve is OFF, step S404 isselected as the next step. If the by-pass route 18 is open, step S403 isselected where the control signal OFF is supplied to the by-pass valve17 for closing the by-pass route 18, and step S404 is selected next.

In step S204, the correlation function of the output signals from thefirst and second air/fuel ratio sensors 6 and 7, one located before theupper stream catalyst 9 and the other located after that, is calculated.In case that step S401 concludes that the current operation status isnot located in the diagnosis region for the upper stream catalyst 9,step S405 is selected as the next step, where further whether theoperation status is located or not in the diagnosis region for the downstream catalyst 10 is judged. In case that the diagnosis region for thedown stream catalyst 10 is established, step S406 is selected furtherand whether the by-pass valve 17 is ON, that is, the by-pass route 18 isopen or not is judged. In case that the by-pass valve 7 is OFF, stepS407 is selected as the next step, where the control signal 0N issupplied to the by-pass valve 17 for opening the by-pass route 18 duringthe diagnosis operations, and step S408 is selected next. In step S408,the correlation function of the output signals from the first and thirdair/fuel ratio sensors 6 and 8 or from the second and third air/fuelratio sensors 7 and 8 is calculated.

And furthermore, in step S409, the correlation function value calculatedin step S408 for anyone of the catalysts is compared with thepredetermined degradation judging level. In case that the calculatedcorrelation function value is proved to exceed the degradation judginglevel, what is concluded is the fact that the catalyst is degraded, andnext in step S410, this status is displayed, for example, reported tothe driver by lighting the alarm signal.

If step S405 concludes that the diagnosis region for the down streamcatalyst 10 is not established, step S410 is selected for going back tothe initial step of the flowchart and restarting the control program.

In addition, in step S411, the judging result is stored in the memory.According to this embodiment, as the individual catalysts including theupper stream catalyst 9 and the down stream catalyst 10 can be treatedas a single catalyst, the diagnostic operation for each catalyst can beindependently performed easily.

As well understood from the above description, in spite that thediagnostic apparatus of the exhaust gas clarification apparatus of thepresent invention is applied to the exhaust gas clarification apparatushaving a plurality of catalysts, this diagnostic apparatus enables theindividually independent diagnosis of each catalyst by estimating thecorrelation function of the output signals from the air/fuel ratiosensors located before and after the individual catalyst in thedistinctive operation region for the individual catalyst.

What is claimed is:
 1. In an internal combustion engine having means fordetecting operation status thereof and an air/fuel ratio control meansfor regulating amount of fuel injection in response to the detectedoperation status so as to keep air/fuel ratio in the exhaust gas thereofat a predetermined value, a diagnosis device for an exhaust gasclarification arrangement including a first exhaust gas catalyst unitdisposed at an upstream side and a second exhaust gas catalyst unitdisposed at a downstream side in an exhaust gas passage in the internalcombustion engine, comprising:a first air/fuel ratio sensor located atan upstream side of the first unit in the exhaust gas passage; a secondair/fuel ratio sensor located between the first and second units in theexhaust gas passage; a third air/fuel ratio sensor located at adownstream side of the second unit in the exhaust gas passage; a firstdiagnostic means for performing diagnosis of the first unit based onoutput signals from said first and second air/fuel ratio sensors; and asecond diagnostic means for performing diagnosis of a combined unit ofthe first and second units based on output signals from said first andthird air/fuel ratio sensors, wherein said first diagnostic means isconfigured to perform the diagnosis of the first unit under a firstoperation region of the internal combustion engine, and said seconddiagnostic means is configured to perform the diagnosis of the combinedunit under a second operation region of the internal combustion engineof which load is larger than that of the first operation region.
 2. Thediagnosis device according to claim 1, wherein the diagnosis devicefurther comprises a third diagnostic means for performing diagnosis ofthe second unit based on the diagnostic information from said firstdiagnostic means of the first unit and from said second diagnostic meansof the combined unit.
 3. The diagnosis device according to claim 2,wherein the third diagnostic means includes means for estimating adegree of deterioration of the second unit with reference to a data mapusing the diagnostic information from said first diagnostic means of thefirst unit and from said second diagnostic means of the combined unit astwo variables on a rectangular coordinate system.
 4. The diagnosisdevice according to claim 1, wherein clarification capacity of thesecond unit is larger than that of the first unit by a predeterminedamount, and the diagnostic device further comprises a third diagnosticmeans for performing diagnosis of the second unit based on outputsignals from said first and third air/fuel ratio sensors and/or outputsignals from said second and third air/fuel ratio sensors.
 5. Thediagnosis device according to claim 1, wherein the first diagnosticmeans includes a first correlation function calculation means forcalculating a correlation function between the output signals from saidfirst and second air/fuel ratio sensors, and said second diagnosticmeans includes a second correlation function calculation means forcalculating a correlation function between the output signals from saidfirst and third air/fuel ratio sensors.
 6. The diagnosis deviceaccording to claim 1, wherein the diagnosis device further comprisesmeans for notification of a diagnosis result, and means for storing thediagnosis result.
 7. In an internal combustion engine having means fordetecting operation status thereof and an air/fuel ratio control meansfor regulating amount of fuel injection in response to the detectedoperation status so as to keep air/fuel ratio in the exhaust gas thereofat a predetermined value, a diagnosis device for an exhaust gasclarification arrangement including a first exhaust gas clarificationunit disposed at an upstream side and a second exhaust gas clarificationunit disposed at a downstream side in an exhaust gas passage in theinternal combustion engine comprising:a by-pass passage configured toby-pass the first unit; a by-pass valve configured to switch exhaust gasflow between through the first unit and through the by-pass passage; afirst air/fuel ratio sensor located at an upstream side of the by-passvalve; a second air/fuel ratio sensor located at one of an upstream sideand at a downstream side of the outlet of the by-pass passage in theexhaust gas passage; a third air/fuel ratio sensor located at adownstream side of the second unit in the exhaust gas passage; a firstdiagnostic means for performing diagnosis of the first catalyst unitbased on output signals from said first and second air/fuel ratiosensors when the by-pass valve is switched to close the by-pass passageto flow the exhaust gas through the first unit; and a second diagnosticmeans for performing diagnosis of the second catalyst unit based on oneof output signals from said first and third air/fuel ratio sensors andoutput signals from said second and third air/fuel ratio sensors whensaid by-pass valve is switched to open said by-pass passage to flow theexhaust gas through said by-pass passage, wherein said first diagnosticmeans is configured to perform the diagnosis of said first unit under afirst operation region of the internal combustion engine, and saidsecond diagnostic means is configured to perform diagnosis of the secondunit under a second operation region of the internal combustion engineof which load is larger than that of the first operation region.
 8. Thediagnosis device according to claim 7, wherein, the diagnosis devicefurther comprises a means for notification of the diagnosis result andmeans for storing the diagnosis result.